Ballasts are circuits used to drive gas discharge lamps, such as fluorescent lamps. Ballasts adjust the lamp current by adjusting the driving frequency. The driving frequency can be adjusted by using analog voltage-to-frequency converters.
Analog ballasts have a large number of external capacitors and resistors to control various parameters, such as a pre-heating time, soft-start time, minimum and maximum driving frequencies, and running frequencies.
Some ballast circuits lower the number of external capacitors and resistors by including digital controllers. Some digital controllers are capable of identifying the type of the driven lamp. Other circuits use the digital controllers to communicate remotely with an external controller. But these digital controllers are still embedded in an analog design.
Ballasts utilizing digital controllers also lack some of the functionalities of regular analog ballasts. For example, in some described ballasts there are no intelligent pre-heating features. Other digital ballasts do not have pre-heating features, or lamp-fault protection, or dimming features. Ballasts, which do describe dimming features, still employ analog circuitry. In some described ballasts, optical couplers are used, again including analog designs.
Among the digital ballasts some designs do not provide short-circuit or load-fault protection. Some have complex pre-heating circuitry with additional auxiliary windings. Also, the external communication involves the high voltage line, an undesirable feature.
Ballasts for automotive applications, such as full-bridge inverters, specialize in 12V voltage applications. Further, they do not describe pre-heating features.
Some designs use digital control functions for specific purposes only, such as identifying the type of the lamp, which is being driven by the ballast. These circuits often have both digital and analog aspects, thus they are not fully digitally controlled.